Transmitting a basic television signal adhering to the National Television System Committee (NTSC) standard requires sending an audio signal to accompany a video signal by frequency-modulating an audio carrier located 4.5 MHz above a video carrier.
Conventional analog television (TV) receivers employ analog components to separate the audio signal from a received NTSC TV signal before demodulating and decoding the audio signal. Such analog components include surface acoustic wave (SAW) filters, amplifiers, automatic gain control (AGC) circuitry and automatic frequency control (AFC) circuitry.
As is known in TV receiver design, a TV signal is received in the radio frequency (RF) portion of the electromagnetic spectrum and converted to an intermediate frequency (IF) by components collectively referred to as a front end tuner. The video signal and audio signal are then, typically, separated at the output of the front end tuner, that is, while the TV signal is in the IF. Subsequently, each signal (audio, video) is demodulated separately by using separate filters and amplifiers.
It is known to perform the demodulation in the digital domain to take advantage of digital signal processing techniques. However, because such demodulation is signal specific, the TV signal is typically filtered by distinct analog filters to simultaneously yield an audio signal and a separate video signal. The separated audio signal and video signal are then processed by corresponding analog-to-digital converters before separate digital demodulation.
The disadvantages of using analog components to separate the audio signal from the received NTSC TV signal are numerous. The analog approach to TV receiver design may be considered to compromise performance due to a lack of precise analog signal filter, analog detector and analog control circuit design. In particular, poor separation of the audio signal and the video signal may cause mutual interference between demodulated audio and video signals. Furthermore, reducing power consumption of the TV receiver may be difficult.
Clearly, an improved design is required for a receiver of an analog television signal, where the improved design provides more precise separation of the audio signal and the video signal from the received TV signal.